There is known a vehicle antitheft device that disables a start of an engine at a set mode for a vehicle theft preventing operation of the a vehicle and enables a start of the engine by canceling the set mode when a key is inserted into an ignition key cylinder and a conduction of an ignition switch is detected in the above-mentioned state (For example, refer to Japanese Laid-Open Patent Application No. 7-156754). In such a vehicle antitheft device, when a normal starting operation is performed, a theft preventing function is canceled. For this reason, according to the above-mentioned conventional antitheft device, it is avoidable that the situation where the theft preventing function operates due to a normal starting operation is avoidable.
By the way, there exists an ignition system that performs an engine start when a collation between an in-vehicle equipment attached to a vehicle and a portable equipment carried by a vehicle driver is completed through a radio communication and an operation of an engine start switch provided in the vehicle is performed. In such as system, it is required to enable a start of an engine by an ignition switch being conducted even if a key is not inserted into the ignition key cylinder. For this reason, in a vehicle having both the ignition system and the vehicle antitheft device disclosed in the above-mentioned patent document, there may be a situation where the theft preventing function cannot be cancelled even when a normal start operation is performed. Therefore, convenience of a vehicle driver may be deteriorated due to the theft preventing function being activated.